It is often desirable to connect more than two endpoints to a communication, such as to establish a conference call between three or more endpoints, or to eavesdrop on a communication between two endpoints via a service-monitoring entity. Control over the communications paths between the multiple endpoints is often given to one of them. For example, a conference host may be able to selectively mute and unmute individual ones of the conference participants, so that they cannot either speak to the other conferees or listen to what the other conferees are saying. Or, a service-monitoring entity, such as a call-center supervisor, may be able to connect call-recording equipment to the monitored communication at will. If multiple endpoints at a common location are always to be automatically connected to a communication involving any of them, it is sometimes the practice to assign the same virtual address—such as a common extension number—to all of them. Dealing with multiple endpoints under a common address simplifies processing of the communications.
People who are hearing-impaired or unable to speak communicate via the telephone network by using specialized text terminals, commonly referred to as a telecommunication device for the deaf (TDD) or a teletypewriter (TTY). A TTY converts symbols (letters, numbers, punctuation, etc.) typed on its keyboard into audio signals that it transmits through the telephone network to another TTY. The receiving TTY converts the audio signals back into symbols and displays them on its display screen. A call between TTYs is established and terminated in the same manner as between telephones: either the TTY emulates the call signaling of a telephone, or the TTY user uses a standard telephone to initiate, dial, answer, and terminate the call, and uses the TTY only for the traffic-exchange portion of the call.
In a Voice over Internet Protocol (VoIP) network, endpoints negotiate communication call-path parameters between themselves as part of the call set-up process prior to establishing the communication path. This is necessary in VoIP systems because, unlike analog telephones on the public network, IP endpoints can generate many different types of specialized packets that vary depending on the media type. Examples include voice packets, text packets, and video packets. As a part of the call set-up process, endpoints specify the types of packets that they are able to accommodate, and then negotiate the types of packets that will be supported during the call. For calls between analog endpoints on the public network and IP endpoints, a similar call set-up negotiation occurs between the IP endpoint and the associated IP/analog gateway.
Looking now at the issue of text transmissions, text terminals such as TTYs do not emit a self-identifying handshake tone or rely on a carrier tone. (In this sense, they are different from computer modems and fax machines.) The devices are silent when not transmitting text. A complicating factor is that, when the user is typing a message, the audio tones emitted by these devices are often not transmitted reliably by the voice-optimized audio channels of VoIP systems. For this reason, VoIP networks transmit text via specialized non-audio mechanisms, such as RFC-2833 protocol (“RTP Payload for DTMF Digits, Telephony Tones and Telephony Signals”).
Because the analog TTY devices are not self-identifying, if a call is placed by a TTY user on the analog public network to the user of an IP telephone, the call set-up negotiation between the IP endpoint and the associated IP/analog gateway is likely to assume erroneously that a non-text voice-only media stream and a non-text voice-only IP endpoint will be satisfactory. The result will be that the user of the IP endpoint will know that he or she has received a call from a TTY user (because distorted TTY tones will be audible on the handset), but will be unable to respond because the IP endpoint is unable to receive or transmit text packets.